1. Field of the Invention
The present invention relates to a sander, and more particularly to a sander having an abrasive disc which performs a dual motion including a revolving motion around its axis and an eccentric rotational motion or an orbital motion.
2. Description of the Prior Art
A conventional sander includes a motor disposed within a body, a spindle rotatably driven by the motor, a connecting shaft rotatably mounted on the spindle and having a rotational axis displaced from the axis of the spindle, and an abrasive disc fixedly mounted on the connecting shaft. A sun gear is fixed to the connecting shaft coaxially therewith. An internal gear is fixed to the body and is in engagement with the sun gear. The number of teeth of the internal gear is greater than that of the sun gear. With such a conventional sander, the abrasive disc may perform a dual motion including a revolving motion around the connecting shaft and an orbital motion or an eccentric rotational motion around the spindle.
Here, the rotation of the spindle is reduced by the sun gear and the internal gear and is transmitted to the connecting shaft of the abrasive disc. Since the abrasive disc is forcibly rotated through engagement of the sun gear with the internal gear, the abrasive disc has a larger momentum. Therefore, the amount of abrasion of a work tends to become greater, and the sander cannot be used to abrade a work made of soft material. Further, the sander involves a disadvantage that it cannot be used for finishing abrasion.
In order to solve this problem, U.S. Pat. No. 4,727,682 proposes a grinder having an internal gear which is movable in an axial direction relative to a sun gear, so that the internal gear is disengaged from the sun gear. The abrasive disc thus becomes free to rotate, and therefore, the momentum of the abrasive disc becomes smaller when the abrasive disc is pressed on a work. U.S. Pat. No. 4,759,152 discloses an internal gear which is removable from the bottom of a body of a grinder, so that an abrasive disc becomes free to rotate as in U.S. Pat. No. 4,727,682.
In case of the grinder of U.S. Pat. No. 4,727,682, since the internal gear is moved to engage and disengage from the sun gear through its axial movement, an extra space is required in the body to permit such axial movement of the internal gear. This may result in increased height of the whole grinder, and therefore, the operability of the grinder is degraded. Further, when the internal gear is moved to engage the sun gear, the teeth of the internal gear may abut on the teeth of the sun gear in the vertical direction. Therefore, this construction has a disadvantage that the internal gear cannot smoothly engage the sun gear and that the teeth of these gears may be damaged.
In case of the grinder of U.S. Pat. No. 4,759,152, a troublesome operation is required to remove and remount the internal gear to shift grinding mode. Further, this construction has a disadvantage that the bottom of the body must have a particular structure for permitting such removal and remounting operation of the internal gear.
Additionally, with both the grinders of the above prior art U.S. patents, since the grinding disc becomes entirely free to rotate when the internal gear is disengaged from the sun gear, the grinding disc idly revolves and the revolving speed of the grinding disc reaches substantially the same rotational speed as the spindle. Thus, the grinding disc revolves at high speed, resulting in that an abrasive sheet attached to the grinding disc may be scattered and that an excellent finishing surface of a work may not be obtained when the grinding disc is applied on the work.